State of Health? Or Paranoia?

She points the gun and I hand over my keys and wallet, no questions asked. "Don't worry," the woman says. She isn't trying to rob me. She's trying to save me.

I believe her. First, the "gun" is hardly intimidating. It's like a prop from a 1950s sci-fi flick—big, gray, and boxy. More telling is where she's aiming it: not at me but at the items I've just surrendered.

The weapon is an x-ray fluorescence analyzer, and the woman holding it is Caroline Cox, research director for the Center for Environmental Health in Oakland, California. She says the analyzer is used to measure the potential toxicity of virtually any consumer product—from wallets, keys, and gym bags to fishing rods, hose nozzles, and gas-grill valves. When the electrons are done dancing, a display reveals whether or not the items contain chemicals at levels deemed toxic under a California law called Proposition 65.

And if a product fails the test? There had better be a warning label on it or its packaging; otherwise the law empowers anyone acting in the public interest to take out the big guns: lawsuits, potential fines, and in extreme cases, efforts to shut down production.

So now Cox pulls the trigger and. . .nothing. She chuckles. "The only thing you see when the machine is in action is this light," she says, pointing to a little indicator on the top. Indeed, in a moment it glows yellow, followed shortly by a set of numbers that flash on a small screen, looking just like a radar gun.

The wallet passes easily. No worries there, unless having an empty billfold is a crime. Next Cox zaps my keys. The light flashes. The numbers whir. Uh-oh.

I'd seen the Prop 65 warnings before, of course. And it's likely you have too. Go to a Walmart and pick up a flashlight, a padlock, a set of dishes, even Christmas lights, and you'll read the clearly printed labels, as ominous as the warning on a pack of smokes: This product contains a chemical known to the state of California to cause cancer. (Another version warns of birth defects or other reproductive harm.)

Maybe the words will register. Maybe not. Maybe you'll even hesitate for a beat. Then, in all likelihood, you'll buy the thing anyway, tear open the packaging, and begin to handle and perhaps eat off something said to be toxic.

And why shouldn't you? After all, this is California. Land of tree-hugging paranoia over the latest environmental bogeymen.

Except. . .what if they're right? What if danger really does lurk in the long list of seemingly innocuous products that carry the warning mandated by state law? What then? I pondered that question as Cox handed back my keys with a sorry-to-inform-you grin. (You could help ward of the Big C with these 3 Cancer-Fighting Foods You're Not Eating.)

When voters overwhelmingly passed Prop 65 in the mid-1980s, its bland-sounding name—the "Safe Drinking Water and Toxic Enforcement Act of 1986"—gave little indication of the sweeping effect it would have on virtually every product made or sold in California, or the controversy that would follow it.

That's because planted within the hugely popular ballot initiative—it passed with more than 60 percent of the vote—was a provision that not only protected the water Californians used but also required companies to "reformulate" or brand with a scarlet letter any products containing chemicals that could cause cancer, birth defects, or reproductive harm.

The California chemical warning was born.

Proponents of Prop 65 saw the law as revolutionary, a way to persuade businesses to do something they are often loath to do: change the very makeup of their products (or else cop to the fact that those products contain harmful ingredients). When new threats emerge, such as BPA and phthalates, the law evolves with the science. From an initial set of 200 toxins, Prop 65 now lists more than 800 chemical enemies, says David Roe, a lawyer who was the senior legal counsel for the Environmental Defense Fund for more than two decades. (How worried do you need to be about BPA? Click here to find out.)

"It has worked remarkably well," says Roe. "Go into any hardware store and you'll find over a hundred products on the shelves that have been changed to take a toxic substance out of them because of Prop 65."